1. Field of the Invention
The present invention relates to electrolysis of an aqueous alkali metal halide solution using an ion exchange membrane, more specifically to a method for preventing an increase of chlorates contained in an alkali metal hydroxide liquor produced by said electrolysis.
2. Description of Prior Art
Recently, an alkali metal halide solution has been electrolysed on an industrial scale by so-called ion exchange membrane method. In such an ion exchange membrane method, it is general to circulate brine as in a mercury electrolysis method. That is, as shown by a schematic block diagram of FIG. 1, brine is adjusted in concentration and the like by a brine adjustment tank (1), then supplied into an anode compartment of a cell. Depleted brine after electrolysis is removed from the anode compartment (2), dissolved chlorine contained in the depleted brine is eliminated thoroughly by a dechlorination tank (3), then introduced to a salt dissolving tank (4) where crude brine is prepared. The crude brine is purified via a primary purification equipment (5) and a secondary purification equipment (6) to become a purified brine.
When ion exchange membrane electrolysis is continued according to the foregoing flow system, a phenomenon occurs that an amount of chlorates contained in catholyte increases. The increased chlorates in the catholyte (e.g., sodium hydroxide liquor) accelerates corrosion of materials of an evaporator, thereby resulting in an increase of impurities such as nickel, chromium and the like in a product. Moreover, some users of the product (e.g., caustic soda) may suffer from a disadvantage in unit consumption and troubles such as corrosion of materials of equipments and therefore the less the content of chlorates is, the better. In the case of caustic soda, the content of sodium chlorate is normally desired to be 10 ppm or less in a 50% NaOH solution.